1. Field of the Invention
The present invention concerns multiple-beam microwave tubes with longitudinal interaction, such as multiple-beam klystrons.
2. Description of the Prior Art
A multiple-beam klystron has N parallel longitudinal electron beams produced by one or more electron guns. The splitting up of a beam into several elementary beams has the advantage of reducing the space-charge effects and of giving a tube with greater efficiency. This also enables the current and power of the tube to be increased or else its operating voltage to be reduced.
Several standard single-beam klystrons can be grouped together in one and the same envelope: in this way a multiple-beam klystron is obtained. The single-beam klystrons are distributed on a ring centered on an axis. This axis is the axis of the multiple-beam klystron. The different electron beams are then parallel to this axis. This construction enables certain elements of standard single-beam klystrons to be used without any notable modification. The beams produced by each of the klystrons are then elementary beams. They go through successive cavities, each cavity being crossed b all the beams.
A standard single-beam klystron is built around an axis which is the axis of the electron beam. A microwave to be amplified is introduced into the order 1 cavity which is on the gun side. This is the input cavity. The last cavity or order m cavity is connected to an external operating element by means of a short transmission line. This is the output cavity. The transmission line is generally positioned crosswise with respect to the axis of the tube. It receives the microwave after amplification. The electron beam is collected in a collector that is coaxial with the axis of the tube. This collector is placed downline of the order m cavity. A focusing device surrounds the cavities. It prevents any divergence of the electron beam.
In a multiple-beam klystron formed by several single-beam klystrons grouped together in one and the same envelope, the focusing device may be common to all the tubes.
In French patent application No. 89 07784, filed on 13th Jun. 1989, the present Applicant has already proposed a klystron type microwave tube having an output coaxial with the collector. According to one embodiment, this application describes a multiple-beam klystron built around an axis. This klystron has, chiefly, a gun producing several electron beams, successive cavities and a collector. Each cavity is crossed by all the beams. The collector located downline of the last cavity is coaxial with the axis of the tube. The last cavity is coupled to a transmission line that surrounds the collector and is coaxial with it. This transmission line is, for example, a coaxial waveguide. The coupling between the output cavity and the transmission line is achieved by at least one coupling aperture.
At low frequency, this tube works appropriately, but once the frequency rises the cavities may contain a large number of modes, for they are oversized in relation to the wavelength transmitted in space.
To overcome this drawback, the dimensions of the cavities must decrease once the frequency is increased. However, these dimensions cannot be reduced sufficiently because of the space taken up by the gun or guns producing the electron beams.
The present invention seeks to overcome this drawback and proposes a multiple-beam microwave tube having groups of cavities. Each group of cavities resonates on only one frequency. Furthermore, this tube can work at high frequency.